The coolest place in Penrith on Sunday was in the Nepean River. Photo: James Alcock

Mr McBurney said Sunday night was the hottest December night in Sydney since 2000, and the warmest night of any month since January 2013.

"We've just had record-breaking heat across the south of the nation, and the trough that has caused that has been gradually approaching, so we had a really hot air mass lingering overnight. There has been no cool change to push it out, so that's really the cause of the warm conditions overnight," he said.

Advertisement

But a cool change was expected to hit the city on Monday morning, causing the temperature to drop throughout the day. Mr McBurney said a significant amount of rain could start sweeping over Sydney from 4pm onwards.

"Once that rain comes in, it will really cool down," he said.

"We could actually see fair bit of rain, which will be good for the fires. Most models right now suggest that by Tuesday, we could see up to 20-30 millimetres of rain across at least northern Sydney, while in some other parts it will be more like 10-20 millimetres."

The chance of showers are forecast each day in Sydney until Boxing Day.

For Christmas Day, the Bureau of Meteorology is forecasting a top of 25 degrees in the city and 27 degrees in the west, with cloudy conditions and a medium chance of showers, most likely in the morning.